The following abbreviations are herewith defined, at least some of which are referred to within the following description of the prior art and the present solution.    C-TAG Customer VLAN TAG    C-VID Customer VID    IEEE Institute of Electrical and Electronics Engineers    PBN Provider Bridged Networks    S-TAG Service VLAN TAG    S-VID Service VID    SPB Shortest Path Bridging    STI Spanning Tree Instances    VID VLAN Identifier    VLAN Virtual Local Area Network
The Shortest Path Bridging (SPB) proposal standardized in the IEEE relies on multiple spanning trees where each bridge has a dedicated spanning tree over which all of the other bridges can be reached on a shortest path (see “Shortest Path Bridging,” IEEE P802.1aq/D0.3, May 9, 2006—the contents of which are incorporated by reference herein). A network implementing this shortest path bridging standard has a requirement that every bridge injecting traffic (or frames) into a SPB domain must transmit this traffic over their respective dedicated source routed spanning tree. Further, these bridges to avoid loops have another requirement where they must forward this traffic over the same dedicated spanning tree while the traffic is within the SPB domain of the network.
These requirements have been graphically illustrated in the network 100 shown in FIG. 1 where a spanning tree that is rooted at bridge A uses VLAN #1 (see lines 102a) and a spanning tree that is rooted at bridge E uses VLAN #2 (see lines 102b). Also in accordance with these requirements, the traffic entering through bridge A and E must be respectively mapped to the VLAN #1 and VLAN #2 so it can be appropriately forwarded through the SPB domain 104. This type of mapping is known in the field as VLAN classification or VLAN translation. In this example, the SPB domain 104 is made-up of bridges B, C, D, F, G, H, and I and it could be assumed that each of these bridges B, C, D, F, G, H, and I use a different forwarding scheme or VLAN mapping scheme than the root bridges A and E.
To implement these requirements, a new way of mapping VLANs to spanning tree instances (STIs) was proposed by Norman Finn in an IEEE presentation entitled “Shortest Patch Bridging” on Sep. 22, 2005. Basically, it was proposed that the root bridges A and E transmit traffic with a 12 bit VLAN identifier that may be logically subdivided into an R part, a M part and a C part (see FIG. 2). These three parts are defined as follows:                Root Part (R): specifies which root bridge is used when routing the frame. It identifies the bridge that injected the traffic into the SPB domain.        Multipath Part (M): specifies which set of link cost parameters is used when routing the frame. If there are multiple alternative trees that form a root bridge then this part is used to differentiate among them.        Community Part (C): specifies a particular VLAN. This is the traditional interpretation of the VLAN ID (VID).Note: This 12 bit VLAN identifier which has the 3 bit R part, 4 bit M part and 5 bit C part is just an example associated with the SPB context and it should be appreciated that the 12 bit VLAN identifier can in other contexts be defined to have different parts and field sizes.        
If the 12 bit VLAN identifier is used to identify the root bridge and as such the spanning tree that must be used to forward the traffic then it follows that the root bridge needs to perform a VLAN classification of data frames to support this particular identification. In practice, the root bridge performs this VLAN classification when it receives tagged frames or untagged frames. The root bridge's VLAN classification of untagged frames is not problematical since the needed VLAN tag can be added in accordance with anyone of the following well known tagging schemes:                1. Port based classification: the root bridge has a single PVID configured per port and all frames that enter at this port are tagged with the same PVID value. See, IEEE P802.1Q-REV/D5.0, “Virtual Bridged Local Area Networks,” Sep. 12, 2005 (the contents of which are incorporated by reference herein).        2. Port and Protocol based classification: the root bridge assigns multiple VID values to frames entering on the same port. The differentiation is based on the specific Ethernet Type values set in the frame headers. See, IEEE P802.1Q-REV/D5.0, “Virtual Bridged Local Area Networks,” Sep. 12, 2005 (the contents of which are incorporated by reference herein).        3. S-VLAN translation: the root bridge (in particular provider bridge) has a VLAN translation table configured on specific bridge ports that specify a one-to-one bidirectional mapping between Port VIDs (VID received in incoming frames) and Relay VIDs (VID used for relaying the frames). To implement this scheme, the S-VID (Service VLAN) was defined which is a new VLAN type that is used in PBN networks. This new scheme introduces a VLAN hierarchy, as multiple C-VLANs (Customer VLANs) can be multiplexed into S-VLANs at PBN edge bridges. See, IEEE P802.1ad/D6.0, “Provider Bridges,” Aug. 17, 2005 (the contents of which are incorporated by reference herein).        
However, the root bridge's VLAN classification of tagged frames is problematical since current VLAN translations schemes require the translation of the whole 12 bit VLAN identifier that is a full 12 bit value is translated into another full 12 bit value. As can be appreciated, this operation does not allow the use and/or consideration of logical interpretation of the specific parts of the 12 bit VLAN field. This problem and other problems are addressed by the present solution.